1. Field of the Invention
This invention relates to a method for preparing nonwoven fabrics containing low levels of multiple-component fibers which have latent three-dimensional spiral crimp, mixed with fibers which do not develop spiral crimp wherein the fabric has an improved balance of properties in the machine and cross-directions.
2. Description of Related Art
Nonwoven fabrics comprising laterally eccentric multiple-component fibers comprising two or more synthetic components that differ in their ability to shrink are known in the art. Such fibers develop three-dimensional helical (spiral) crimp when the crimp is activated by subjecting the fibers to shrinking conditions in an essentially tensionless state. Helical crimp is distinguished from the two-dimensional crimp of mechanically crimped fibers such as stuffer-box crimped fibers. Helically crimped fibers generally stretch and recover in a spring-like fashion.
U.S. Pat. No. 3,595,731 to Davies et al. (Davies) describes bicomponent fibrous materials containing crimped fibers which are bonded mechanically by the interlocking of the spirals in the crimped fibers and adhesively by melting of a low-melting adhesive polymer component. The crimp can be developed and the potentially adhesive component activated in one and the same treatment step, or the crimp can be developed first followed by activation of the adhesive component to bond together fibers of the web which are in a contiguous relationship. The crimp is developed under conditions where no appreciable pressure is applied during the process that would prevent the fibers from crimping.
U.S. Pat. No. 5,102,724 to Okawahara et al (Okawahara) describes the finishing of nonwoven fabrics comprising bicomponent polyester filaments produced by conjugate spinning of side-by-side filaments of polyethylene terephthalate copolymerized with a structural unit having a metal sulfonate group and a polyethylene terephthalate or a polybutylene terephthalate. The filaments are mechanically crimped prior to forming a nonwoven fabric. The fabric is rendered stretchable by exposure to infrared radiation while the filaments are in a relaxed state. During the infrared heating step, the conjugate filaments develop three-dimensional crimp. One of the limitations of this process is that it requires a separate mechanical crimping process in addition to the crimp developed in the heat treatment step. In addition, the process of Okawahara requires the web or fabric to be in continuous contact with a conveyor such as a bar conveyor or a pre-gathering slot along spaced lines corresponding to the bars in the bar conveyor or lines of contact where the web contacts the gathering slot, as the product is shrunk or prepared for shrinking. Processing through a pre-gathering slot requires the use of cohesive fabrics that are pre-integrated and cannot be used with the substantially non-bonded nonwoven webs that are used in the process of the current invention. Multiple-line contact with a bar conveyor during the shrinkage step interferes with fabric shrinkage, crimp development, and fiber re-orientation even when the fabric is overfed onto the conveyor.
PCT Published Application No. WO 00/66821 describes stretchable nonwoven webs that comprise a plurality of bicomponent filaments that have been point-bonded prior to heating to develop crimp in the filaments. The bicomponent filaments comprise a polyester component and another polymeric component that is preferably a polyolefin or polyamide. The heating step causes the bonded web to shrink resulting in a nonwoven fabric which exhibits elastic recovery in both the machine direction and the cross direction when stretched up to 30%. Since the length of fiber segments between the bond points varies, pre-bonding of the fabric prior to shrinkage does not allow unimpeded crimp development among all of the filaments since the shrinking stresses are unequally distributed among the filaments. As a result, overall shrinkage, shrinkage uniformity, crimp development, and crimp uniformity are reduced.
Japanese Kokoku Patent Number 8(1996)-19661, assigned to Japan Vilene Co., Ltd., describes nonwoven fabrics containing at least 30 percent of side-by-side latent crimpable fibers which have been hydraulically entangled followed by heat treatment to develop the crimp of the latent crimpable fibers. The hydraulic entanglement of the fibers prior to shrinkage does not allow equal and unimpeded crimp development.
U.S. Pat. No. 3,671,379 to Evans et al. (Evans) describes self-crimpable composite filaments which comprise a laterally eccentric assembly of at least two synthetic polyesters, the first of said two polyesters being partly crystalline in which the chemical repeat-units of its crystalline region are in a non-extended stable conformation and the second of said two polyesters being partly crystalline in which the chemical repeat-units of the crystalline region are in a conformation more closely approaching the length of the conformation of its fully extended chemical repeat-units. The composite filaments are capable of developing a high degree of spiral crimp against the restraint imposed by high thread count woven structures, which crimp potential is unusually well retained despite application of elongating stress and high temperature. The composite filaments increase, rather than decrease, in crimp potential when annealed as a part of the fiber production process. The filaments are described as being useful in knitted, woven, and nonwoven fabrics. Preparation of continuous filament and spun staple yarns and their use in knitted and woven fabrics is demonstrated.
Carded staple webs, including those containing multiple-component fibers, are well known in the art. Fibers in carded webs are characterized by machine direction (“MD”) and cross-direction (“XD”) web axes. Carded webs have a predominance of MD-oriented fibers that yield fabrics having correspondingly enhanced MD and diminished CD tensile strength. Air-laid and spunbonded webs also, in general, tend to favor MD orientation to various degrees depending upon the type of machinery, fiber, and laydown conditions. Cross-lapped carded webs with many layers tend to have a fiber orientation predominantly in the cross direction. There exists a need for providing uniform nonwovens from carded webs and other nonwoven processes that have an improved balance of properties in the machine and cross direction, especially to provide balanced tensile strength as well as uniformity and drape.